1. Technical Field
The present invention relates to the field of semiconductor integrated circuit fabrication. More particularly, the present invention relates to the field of equipment used in high temperature processing of integrated circuits.
2. Description of the Related Art
Integrated circuit fabrication typically involves the use of carriers, otherwise known as "boats", containing batches of wafers. These carriers are then used to transport and hold the wafers inside furnaces as well as other equipment. These carriers are usually made of special materials, such as quartz and silicon carbide, that allow them to be used at the elevated temperatures inside furnaces while the wafers are being processed. The material that these carriers are made from also impacts the quality of the wafers being processed. Some materials are not suitable because they would contaminate the wafers and render the wafers useless.
The equipment used in wafer processing must be extremely clean to avoid contamination of the wafers. Microscopic particles and very small amounts of unwanted chemicals can destroy integrated circuits. The percentage of good integrated circuits from a particular fabrication lot is known in the industry as the yield percentage. It is well known in the art that a small drop in yield can have a very large economic impact.
Human beings are one of the worst sources of contamination in a semiconductor fabrication facility. In order to minimize the contamination of the wafers, it is common to have machines load and unload the individual wafers into the boats.
These machines require strict dimensioning in order to function properly. If the carriers change dimensionally after a period of time due to the stresses of the fabrication processes, they could prove to be unusable in the wafer loading equipment.
Existing materials used for making wafer carriers have less than ideal characteristics. No matter how well a carrier using these materials is made and how clean the material, it still can contaminate the wafers. This is most often caused by impurities imbedded in the material. These impurities can `out-gas`, or release, out of the carriers during high temperature processing.
Polysilicon can be manufactured to extreme cleanliness standards, which lends itself very well to the semiconductor industry. However, it is very brittle and not easily molded. Also, making bonds to polysilicon is difficult. It cannot be easily welded nor can it be easily glued together. Most glues would constitute a source of contamination and cannot withstand the extreme conditions of integrated circuit fabrication.
Another problem with some of the existing carriers is that the material that the carriers are made from actually flows at elevated temperature. The carriers actually can change size over time at elevated temperatures. This can pose a huge problem to wafer handling automation. These carriers are also limited in their useful temperature range. In certain processes, some commonly used materials cannot be used because the carrier would deform to an unusable state rather quickly.